1. Field of the Invention
The present invention relates to micro-mechanical devices for reflective displays and, more particularly, to a reflective display having deformable mirrors.
2. Description of the Related Art
There are significant efforts underway to develop low power, high resolution, xe2x80x9cpaper-likexe2x80x9d displays using either liquid crystals (T. Ogawa et al., xe2x80x9cThe Trends of reflective LCDs for future electronic paperxe2x80x9d, SID ""98 Digest, p. 217) or other technologies. The liquid crystal based approaches generally suffer from low reflectivity and poor contrast ratios. There has recently been a number of publications on using MEM (microelectromechanical) devices for display applications. Projection systems based on arrays of tilting mirrors have been commercialized (See, e.g., J. Sampsell, xe2x80x9cAn overview of the digital micromirror device (DMD) and its application to projection displaysxe2x80x9d, SID ""93 Digest, p. 1012) and projection systems proposed using a micromechanical phase grating (See, e.g., D. Bloom xe2x80x9cThe grating light valve: revolutionizing display technologyxe2x80x9d, SPIE Vol. 3013 (1997) p. 165).
Two types of MEM based direct view displays have also been proposed. In the first (See e.g., E. Stern, xe2x80x9cLargearea micromechanical flat-panel displayxe2x80x9d, SID 97 Digest, p. 230), an array of passively addressed bistable transparent beams are used to control the release of light trapped by total internal reflection. This device uses a back light, and due to the thick optical feed structure (about 4 cm) is not be suitable for portable displays. A second direct view display (See e.g., M. W. Miles, xe2x80x9cA new reflective FPD technology using interferrometric modulationxe2x80x9d, SID 97 Digest (1997) p.71) includes the use of a micromachined deformable optical cavity whose reflected color changes with voltage. The device includes a self-supporting deformable membrane, made of, for example aluminum, and a thin film stack, both residing on a transparent substrate. The self-supporting deformable membrane and the thin film stack act as mirrors for an optically resonant cavity. When a voltage is applied, the deformable mirror collapses and the color of the reflected light is changed. The devices are binary and have hysteresis which allows passive addressing. The color selection of the two states is determined by the optical stack (which contains a conductor) and by the rest height of the deformable mirror. The main disadvantage of such a system is that the maximum reflectivity is limited. For a narrow color band, the peak reflectance can be about 80%. If an 80% reflectivity is assumed for the whole Red, Green, and Blue bands, a triad pixel structure, and an 80% aperture ratio, the maximum white reflectivity would be about 21%. For a paper-like display, a reflectivity of 40% or more is necessary.
A type of display, referred to as xe2x80x9celectroscopic displaysxe2x80x9d, have been described by T. S. Te Velde et al. which are bistable and have an improved reflectivity compared to the interferrometric modulation displays described above. (See Te Velde et al., xe2x80x9cA family of electroscopic displaysxe2x80x9d, Society of Information Display 1980 technical digest, p. 116-117 and the following U.S. Pat. Nos. 4,178,077, 4,519,676, 4,729,636.) The article entitled, xe2x80x9cA family of electroscopic displaysxe2x80x9d. (hereinafter Velde), describes an electroscopic fluid display where a plate or grid which is reflective and is movable is sealed with a glass plate and filled with a nonconducting black or other colored solvent. If the penetration depth of the incident light in the solvent is much smaller than the thickness of the cell, than when the white grid is located near the bottom plate, the grid will not be visible and the cell will appear black. However, when the white grid is attracted to the front side, the white grid will be visible and the cell will appear white.
Two possible configurations are described in Velde, a springy capacitor and a triode. For the springy capacitor, the grid is mechanically fastened to the bottom plate via conductive springs and when a large enough voltage is applied, the springs are stretched and the grid rushes to the upper electrode. This arrangement requires careful cell gap control since the threshold voltage is a function of the cell gap.
In the triode configuration, the springs are made very weak so that mechanical forces can be neglected and electrodes on the top and bottom plates are used to electrostatically control the position of the reflective plate. In U.S. Pat. No. 4,178,077, a triode configuration is described where electrostatic forces are used to control a movable electrode in an opaque liquid. A fabrication process is also described which uses an underetching process where apertures in a second layer provide access for the etchant to the first layer. This requires a timed etch to leave portions of the first layer in place to support the second layer. In U.S. Pat. No. 4,519,676, a triode configuration is again described, but with the resilient elements below the display part to increase the aperture ratio. A more complicated fabrication process is described which again uses timed underetching. In U.S. Pat. No. 4,729,636, engaging points are formed between the movable electrode and its engaging surface to improve the response time by allowing liquid flow in and out during closure and release. The triode configuration is complicated and requires electrical contacts for addressing to be formed on both top and bottom plates. Both the triode and springy capacitor (when fastened to the bottom plate) require precise cell gap control since the threshold voltage depends on the cell gap. The fabrication processes described require the etching step to be stopped by a certain time or the first layer will be fully removed and the second layer will no longer be attached to the substrate.
For a high information content display, such as one for an 8.5 inch by 11 inch sized display with 150 dot per inch resolution, it is advantageous to integrate some of the addressing electronics on the display itself to reduce cost and improve yield. For the display size described above, approximately 1,275 gate line and about 1,650 data line connections and driver chip outputs are needed. If the display technology used can also provide switches, the row selection circuits (i.e., shift register) and data driver demultiplexing circuits may be made with the display and greatly reduce the number of connections and drivers. (See xe2x80x9cSilicon light valve array chip for high resolution reflective liquid crystal projection displaysxe2x80x9d, by J. L. Sanford et al., IBM J. Res. Develop., Vol. 42 No. 3/4, May/June 1998, pp. 347-358, incorporated herein by reference.)
Therefore, a need exists for a portable display having high reflectivity and a high contrast ratio. A further need exists for a display which permits switches to be fabricated at the same time as the display device. A still further need exists for a method for fabricating the display device and switches in an efficient and economical manner.
A display device, in accordance with the present invention includes a transparent substrate and an array of pixels formed on the substrate, each pixel including a transparent electrode and a deformable member electrically actuated between a first state and a second state, wherein in the first state a liquid including a dye is disposed in a gap between the transparent electrode and the deformable member and wherein in the second state the deformable member contacts the transparent electrode to define an area of contact thereby closing the gap such that the liquid is substantially removed between the deformable layer and the transparent electrode in the area of contact. A plurality of switches is formed on the substrate for supplying control signals to the array of pixels to selectively actuate the deformable members of the pixels, wherein each switch comprises an actuating member movable between an active state and an inactive state, whereby in the active state any control signal supplied to the switch passes through the switch, and in the inactive state any control signal supplied to the switch is prevented from passing through the switch.
In alternate embodiments, the deformable member may include a reflective surface which contacts an insulation layer over the transparent electrode in the second state. The dye may be black such that light is reflected from the area of contact when the deformable member is in the second state and light is absorbed in the gap when the deformable member is in the first state. The dye may include Sudan Black or Naphthol Blue Black. The deformable member may include a light absorbent surface which contacts the transparent electrode in the second state. The dye may be white such that light is reflected from the gap when the deformable member is in the second state and light is absorbed in the area of contact when the deformable member is in the first state. The deformable member is bistable having a hysteresis such that only the first and second states are permitted. Alternatively, the deformable member may be adjustable between a plurality of states thereby adjusting the gap to provide reflected light on a grey scale (i.e., various intensities). The switches preferably include microelectromechanical switches and are formed simultaneously with the display elements.
In other embodiments, the deformable member is preferably actuated on hinges integrally formed with the deformable member. The device may include an active area which may include a first seal region for maintaining the liquid in the active area. The display device may further include a second seal region for maintaining an inert gas therein between the first seal region and the second seal region such that the plurality of switches exist in the inert gas. The transparent electrode may form a data line for controlling the pixels and the deformable member may form a gate line for controlling the pixels such that voltage differences provided by control signals between the gate line and the data line provide a force for actuating the deformable member. The plurality of switches may be formed on the substrate for supplying the control signals on gate lines and data lines to the array of pixels to selectively actuate the deformable members of the pixels. A shift register may be included using a portion of the plurality of switches, the shift register is employed for addressing the gate lines. Switches may also be employed for demultiplexing the data lines to reduce the number of data driver chips and electrical connections needed.
Another display device, in accordance with the invention includes a substrate and an array of pixels formed on the substrate, each pixel including a transparent substrate and a deformable member electrically actuated between a plurality of states. In each of the states, a liquid including a dye is disposed in a gap between the transparent substrate and the deformable member in an active area and wherein the gap is adjustable according to voltages applied to and stored by each pixel thereby reflecting light from the active area according to a grey scale (i.e., by varying the intensity of the reflected light).
In alternate embodiments, the deformable member may include a reflective surface for reflecting light through the transparent substrate and wherein the dye is black. The deformable member may include a light absorbent surface for absorbing light through the transparent substrate and wherein the dye is white. The deformable member is preferably actuated on hinges integrally formed with the deformable member. An active device area may be included, and a first seal region may also be included for maintaining the liquid in the active area. The display device may include a second seal region for maintaining an inert gas therein between the first seal region and the second seal region such that the plurality of switches exist in the inert gas. A shift register may be included, and a portion of a plurality of switches may be used to construct the shift register. The shift register is preferably for addressing gate lines which are used to activate switches in each pixel to connect the data lines to storage capacitors in each pixel. Switches may also be employed for demultiplexing the data lines to reduce the number of data driver chips and electrical connections needed. Data lines are independent of gate lines.
A method for fabricating a display device includes providing a top plate, patterning a transparent electrode in an active region on the top plate, forming an insulating layer on the transparent electrode, patterning a low reflectivity conductive material to form a source electrode, a gate electrode and a drain electrode on the insulating layer outside the active area, patterning a sacrificial layer, patterning a metal layer to form deformable members having a gap between the metal layer and the transparent electrode in the active area and switches outside the active area such that upon activating the gate electrode an electrical connection is made between the source electrode and the drain electrode, removing the sacrificial layer and filling the gap with a liquid including a dye such that in a first state of the deformable member the liquid is disposed in the gap between the transparent electrode and the deformable member and wherein in a second state the deformable member contacts the insulating layer over the transparent electrode to define an area of contact thereby closing the gap such that the liquid is substantially removed between the deformable layer and the insulating layer over the transparent electrode in the area of contact.
In alternate methods, the step of patterning the sacrificial layer preferably includes the steps of forming a via hole through the sacrificial layer to the source electrode and forming a tip feature hole over the drain electrode such that upon patterning the metal layer a cantilevered conductor is attached to the source electrode and includes a tip feature for contacting the drain electrode. The step of forming the source electrode, the gate electrode, the drain electrode and a black matrix concurrently from a low reflectivity conductive material may also be included.
Another method for fabricating a display device includes the steps of patterning a black matrix layer on a transparent substrate, depositing a first insulation layer on the patterned black matrix layer, patterning a transparent conductor layer on the first insulation layer, depositing a second insulation layer on the transparent conductor layer, depositing a sacrificial layer on the second insulation layer for forming a gap of a predetermined distance between the second insulation layer over the transparent conductor layer and deformable members, forming openings in the sacrificial layer for providing support points for deformable members, patterning a metal layer to form deformable members and removing the sacrificial layer to provide the gap.
In alternate methods, the step of filling the gap with a liquid including a dye such that in a first state of the deformable member the liquid is disposed in the gap between the transparent electrode and the deformable member and wherein in a second state the deformable member reduces the gap between the second insulation layer over the transparent electrode and the deformable member such that the liquid is substantially removed between the deformable member and the second insulation layer over the transparent electrode is preferably included. The sacrificial layer may include copper and the step of removing the sacrificial layer may include the step of removing the sacrificial layer by a wet etch process. The deformable members include deformable mirrors.
Yet another method for fabricating a deformable display device includes the steps of patterning a transparent conductor layer in an active area of a transparent substrate, forming an insulation layer over the transparent conductor layer, patterning a conductive black matrix layer on the insulation layer outside the active area, the black matrix layer used for forming a drain electrode for switches, providing a source electrode-and a gate electrode for switches by patterning one of the black matrix layer and the transparent conductor layer outside the active area, patterning a sacrificial layer for forming features in the sacrificial layer for providing support points for the deformable member""s connections through the sacrificial layer (including tip features for the switches) and patterning a metal layer on the sacrificial layer to form the deformable members and support points for the deformable members, the deformable members including deformable display members in the active area and switches outside the active area, removing the sacrificial layer to provide a predetermined gap between the insulation layer over the transparent conductor and the deformable display members and to provide cantilevered conductors for the switches, the cantilevered conductors attaching to the source electrode and including a tip feature for contacting the drain electrode when the gate electrode is activated.
In other methods, the step of patterning a sacrificial layer may include the steps of forming a via hole through the sacrificial layer to the source electrode and forming a tip feature hole over the drain electrode such that upon patterning the metal layer the cantilevered conductor is attached to the source electrode and includes the tip feature for contacting the drain electrode. The sacrificial layer may include a conductive top portion and a lower insulating portion and may further include the steps of forming dimples in the top portion and in a portion of the bottom portion for forming the cantilevered conductors for switches and forming openings through the top and bottom portions to form vias through the sacrificial layer. The conductive top portion may include copper and the lower insulating portion may include polyimide, the method may further include the steps of removing the top portion with a wet etching process and removing the lower portion by a plasma etching process. The deformable display members preferably include deformable mirrors. The method may also include the step of filling the gap with a liquid including a dye such that in a first state of the deformable display member the liquid is disposed in the gap between the transparent electrode and the deformable display member and wherein in a second state the deformable display member reduces the gap between the insulation over the transparent electrode and the deformable display member such that the liquid is substantially removed between the deformable display member and the transparent electrode.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.